1. Field of the Invention
This invention relates to a Fourier transform hologram, and more particularly to a volume holographic memory-based optical information-recording/reproducing apparatus.
2. Description of the Related Art
When parallel light perpendicularly impinges on an image formed on a plane as a dot pattern of light and dark having a transmittance distribution, the parallel light is diffracted intensely in a direction perpendicular to the structure thereof.
In general, an image can be considered to be an overlap of various spatial frequency components in different directions, just as an electric signal or an acoustic signal which varies with time can be considered to be decomposed into various sinusoidal wave components. Mathematically, distribution of the spatial frequency components can be obtained by calculating a two-dimensional Fourier transform.
Optically determining an angular distribution of amplitude of diffracted light which is diffracted under the Fraunhofer's law by causing uniform parallel light to impinge on an image is equivalent to mathematically calculating the two-dimensional Fourier transform of amplitude transmittance of the image. A Fourier transform hologram is formed by causing diffracted light from an image illuminated by coherent parallel light, i.e. a signal light to pass through a Fourier transform lens disposed apart from the illuminated image by a focal distance thereof, to cause an image as a distribution of the signal light to be formed on a focal surface or Fourier surface, then causing interference between the distribution of the signal light resulting from the Fourier transform and a coherent reference beam, and recording the distribution of the signal light as interference fringes on a photosensitive material applied on a flat plate.
A wavefront recorded in the Fourier transform hologram corresponds to an image transformed through Fourier transform, so that it is required to perform inverse Fourier transform to reproduce the image from the wavefront. The inverse Fourier transform is performed by reproducing the diffracted light by illuminating the planar Fourier transform hologram with the identical reference beam and converging the diffracted light by means of the Fourier transform lens. Thus, the amplitude transmittance distribution of the original image is reproduced on the Fourier surface.
As described above, the planar Fourier transform hologram is capable of not only storing a hologram within a limited space but also enhancing redundancy of a record through dispersion of information in space by Fourier transform.
Another type of Fourier transform hologram is a volume hologram having a larger thickness than that of such a planar recording medium described above. Generally, the volume hologram is capable of attaining an enhanced diffraction efficiency, so that it has an advantage in recording bulk information. In the volume holographic memory, information is stored in units of two-dimensional image pages dispersed in a three-dimensional space of the recording medium.
In recent years, a recording medium, such as a photo refractive crystal of lithium niobate (LN), has drawn attention as a volume holographic memory which is capable of recording a three-dimensional interference pattern therein as spatial changes in refractive index of the recording medium.
This photo refractive effect utilized in the recording medium is a phenomenon in which electric charge generated by optical pumping moves within the crystal to form a space electric field, and the space electric field causes a linear electro-optical effect, i.e. the Pockels effect, to change the refractive index of the crystal. For example, in a ferro-electric crystal having the photo refractive properties, a change in refractive index occurs in response even to a fine optical input pattern generally having 1000 lines or more per millimeter therein. Further, the photo refractive effect is generated in real time at a response speed in the order of microseconds to seconds in dependence on the material of a crystal. Therefore, research has been carried out for various applications of the photo refractive crystal as a real-time holographic medium which does not require development of an image.
In recording digital data in the holographic memory, digital data is converted to a dot pattern image of light and dark, for example, on a plane of a panel of a transmissive thin film transistor liquid crystal display (hereinafter referred to as "LCD") by using spatial optical ON/OFF signals, and interference between diffracted light from the image data, i.e. a signal beam, and a coherent reference beam is caused, to record the interference pattern in a rectangular parallelepiped recording medium. In reading the digital data from the holographic memory, the image of the dot pattern is regenerated by irradiating the holographic memory with the same light beam as the reference beam. The regenerated image is received by a photoelectric detector array, and an output signal from the detector array is processed by an electronic circuit to convert the output signal back to the digital data for reading.
The image data is recorded in a portion of the recording medium where the signal beam and the reference beam intersect with each other, so that it is possible to perform space multiple recording by properly shaping a cross section of the reference beam in a manner adapted to a shape of the recording medium. For example, if the reference beam is shaped into a beam having an elliptical cross section having a vertical length of 1 mm and a horizontal length of 4 mm, it is possible to perform multiple recording in a vertical direction, at space intervals of 1 mm. In this case, the signal beam and the reference beam are made coincident at a position for recording.
On the other hand, in conventional volume holographic recording performed by the use of a Fourier transform lens, if interference between a signal beam and a reference beam is caused in the vicinity of a Fourier surface, light intensity becomes saturated, for example, at the center of a 0-order light beam, resulting in a deformed recorded image.
Therefore, interference between the two light beams is caused slightly away from the Fourier surface, so as to prevent light intensity from being saturated. However, this method, in which the light intensity of the signal beam and that of the reference beam are required to be adjusted on the basis of spectral distribution of a recording beam after Fourier transform, is not suitable for multiple recording of various signals having different spatial frequency components.
Further, a CCD image sensor (hereinafter simply referred to as "CCD") and the LCD, each of which uses a matrix of a plurality of charge coupled devices, have been developed in the fields of techniques of image pick-up and image display, respectively, and each required to have a larger open area ratio for improvement of its performance. However, when these devices are applied in the field of digital volume holography, crosstalk between adjacent pixels is increased due to their high open area ratio, resulting in degradation of a reproduced holographic image.
Still further, conventionally, an apparatus of this kind uses a CCD having a higher open area ratio and is configured such that a brighter reproduced image can be obtained. To this end, a tolerance in positioning is limited to a value equivalent to a distance between adjacent light receivers of the CCD (or several pm or less), which requires high assembling accuracy.
Basically, the CCD is susceptible to crosstalk between adjacent pixels. Therefore, as the light-receiving area is increased to obtain a higher signal level, the crosstalk between adjacent pixels becomes larger.
To overcome this problem, when the charge coupled devices used as light receivers for a digital information-recording/reproducing apparatus, an approach is employed in which one information unit (1 bit to a few bits) is formed by a plurality of pixels adjacent to each other, for example, two or four pixels, for reduction of the adverse effect of crosstalk.
However, this approach suffers from redundancy of information and reduces the density of recording.
Moreover, in multiple recording in which the photo refractive effect is utilized for recording information as diffraction gratings, preceding recorded diffraction gratings are progressively erased as the multiple recording of subsequent diffraction gratings proceeds. An attenuation coefficient of this erasure is referred to as the erasing time constant. It is required that measurement of an erasing time constant be carried out in advance on a medium for use in recording. The relationship in recording time between pages, which depends on the order of recording, is determined based on the erasing time constant. The operation for this determination is referred to as scheduling. Multiple recording is performed following results of the scheduling, whereby a reproduced image having a desirable brightness can be obtained.
However, crystals are different from each other in an optical constant, the response speed, the degree of polarization, the erasing time constant, etc., which makes it difficult to attain homogeneous recording.